1. Field of the Invention
This invention relates to information reading apparatus utilized in a digital copying machine, facsimile device or the like and more particularly improved information reading apparatus capable of improving the accuracy of data read by manuscript reader such as a charge coupled device (CCD) image sensor.
2. Description of the Prior Art
In a conventional manuscript reading apparatus utilized in a digital copying machine, a facsimile device and the like, a manuscript written with a picture image, characters or the like is irradiated with light from a bar shaped light source, and the light reflected from the manuscript is inputted into such solid image sensor as the CCD or the like through an image pick-up lens for converting the inputted light signal into an electric signal. In such manuscript reading apparatus it is necessary to correct so-called shading characteristic caused by (1) decrease in the brightness from the central portion of the light source toward the end portion due to inequality in the light distribution characteristic of the light source, (2) decrease in the brightness of the lens near the peripheral portion thereof due to cosine four power rule (3) difference in the sensitivities of respective bits of the solid state image sensor and (4) contamination of a reflection mirror etc.
For this reason, various shading correction systems have been proposed, one example thereof being shown in FIG. 1.
In a prior art apparatus shown in FIG. 1 and disclosed in Japanese Laid Open Patent Specification No. 19467/1984, at the time of the shading correction, a switch 4 is thrown to a contact A and light reflected from a white plate and utilized as a reference image density data for the shading correction is inputted to a solid state image sensor (CCD) 1. Curve I shown in FIG. 2 shows the output characteristic of the CCD 1 when the reference density data Ri is read. The reference density data Ri photoelectrically converted by CCD 1 is inputted into an amplifier as serial bits to be amplified. The output of the amplifier 2 is converted into a digital signal by an A/D converter 3. The resulting digital signal is written into a random access memory device (RAM) 5.
At the time of scanning the manuscript, the switch 4 is thrown to a contact B. The picture image data obtained by reading the manuscript is applied to one input terminal of a divider 6 through the A/D converter 3 to act as a divident. Curve II in FIG. 2 shows the output characteristic of CCD 1 when any manuscript is read. To the other input terminal of the divider 6 is applied the shading correction reference density data stored in RAM 5 as a divider Ri. Consequently, the divider 6 outputs a quotient Di/Ri thereby correcting the shading of the picture image data of the manuscript.
The shading correction data Ri and the picture image data Di of the manuscript read by the CCD image sensor contain a level, that is a base level Ci (shown by curve III in FIG. 2) which appears when no light is inputted into the CCD image sensor so that the true output levels of the data Ri and Di correspond to the differences between these data Ri and Di and the base level Ci.
For this reason, in the prior art, a true output value has been obtained by offsetting the base level by a fixed level in the amplifier 2.
With the prior art system, however, the reading accuracy of the CCD image sensor was degraded due to (1) variation of the offset voltage caused by temperature change, (2) variation in the offset voltage over a long period and (3) difference of the base levels of respective reading elements.
Further, where a one dimensional CCD image sensor shown in FIG. 3 is used as the manuscript reading apparatus, and where a conventional offset system utilizing an amplifier is used, there are the following defects.
That is, as shown in FIG. 3 the one dimensional CCD image sensor is constituted by a photoelectric converting element array 7, a storage electrode 8 for temporarily storing a resulting charge signal, a shift gate circuit 9 for parallelly transferring the stored signal charge to the CCD shift register 10, a CCD shift register 10 for serially transferring the transferred charge to an output unit and an output unit 11 for alternately converting the signal charges transferred from both shift registers 10 into voltage signals for producing a time series signal. While the signal charges of the odd and even numbered photoelectric converting elements 7 are transferred to independent shift registers 10 and then alternately read out at the output unit 11 as time series signals.
FIG. 4 shows an output waveform of the output unit 11 obtained by synthesizing read out channels of the odd and even numbered picture elements. This output waveform was obtained when a 4 MHz reset pulse RS was applied for returning a floating capacitor in the output unit 11 to an initial state, the output waveform being superposed on a DC voltage of about 6V.
Considering the output waveform shown by a solid line obtained when no light was inputted, it will be noted that there are differences in the output waveforms and the output levels between the odd numbered channels and even numbered channels respectively. The difference thereafter increases to more then 0.05V depending upon the timing setting of A/D conversion.
The output waveform when the storage electrode 8 becomes substantially saturated is shown by broken line. Although the output level decreases about 1.3V compared with a case of no input light, at this time too, there are differences in the output waveforms and the output levels between the odd numbered and even numbered channels just as in the case of no light input. Although the output level has decreased by about 1.3V as compared to the case of no light input, in this case too, there are differences in the output waveforms and output levels between the odd numbered channels and the even numbered channels in the same manner as in the case of no light input.
As above described, in the one dimensional CCD image sensor irrespective of the difference in the dynamic ranges between the odd numbered and even numbered channels, according to the prior art system, the offset is made with the same base level. With such measure, the output signal of the CCD image would not be compensated for correctly so that such measure cannot be used in a reading apparatus required to reproduce the manuscript picture image at a high fidelity.
Furthermore, in the prior art apparatus shown in FIG. 1 where a device constituted by hard logic is used as the divider 6, since the number of calculation steps are large, the calculation takes a long time. Therefore as the circuit is elaborate, a read only memory is generally used as the divider in order to increase the calculating speed and simplify the circuit.
The memory content of the read only memory device (ROM) is selected such that when Di and Ri are inputted to the address terminal of the ROM, the ROM outputs a data Di/Ri or an approximate value thereof.
With this system, however, the number of bits required for the input address signal of the ROM is equal to the sum of the bit numbers of the input signals Di and Ri. Moreover, as there is a limit for the bit number of the output data of existing memory devices, where the bit numbers of the input signals Di and Ri are large the circuit construction becomes large and complicated. For example, assume that each of the input signals Di and Ri has 8 bits, the ROM is required to have an address of 2.times.8=16 bits and an output data of 8 bits. With this bit number, the memory capacity becomes 524K bits, thus complicating the circuit construction, increasing the cost of the apparatus and making it difficult to obtain a high operating speed.